Crude oils have various percentages of atmospheric residuum, sometimes called long residuum, which exhibit a boiling point of from about 340.degree. C. to a final boiling point, generally in excess of 650.degree. C. A "heavy crude" is a crude oil having a high percentage of atmospheric residuum as well as having a high percentage of short residuum. Short residuum, or vacuum residuum, is defined as that portion of the crude oil which has a boiling point of from about 565.degree. C. to the final boiling point of the crude oil.
The short residuum contained in such crudes generally contain relatively high Conradson carbon residue precursors, and/or asphaltenes, as well as, in many cases, high sulfur, nitrogen and metals. Examples of heavy, low-sulfur crude oils include the Western African crudes (such as Cabinda and Takula) and various Pacific Rim crudes such as Daqing). Examples of heavy, high-sulfur crudes include the Venezuelan crudes (such as Boscan, Bachaquero and Merey), Canadian crudes (such as Cold Lake and Lloydminster) and Mexican crudes (such as Maya).
Ethylene and propylene--basic intermediates in the production of polyolefins--are typically obtained by thermal steam cracking (pyrolysis) of natural gas liquids (ethane, propane and butane) or petroleum distillates (gasoline, condensates, naphtha and gas oil). As the worldwide demand for such light olefins increases, it has become highly favorable to use heavier feedstocks. In the last twenty years, processes have been developed to utilize higher boiling point distillates as olefin feedstocks.
In general, the use of higher boiling point olefin feedstocks require increased capital investment in the olefins plant. As the boiling point of naturally-occurring feedstock components rises, the olefins cracking yield patterns shifts from 70+ weight percent ethylene (for an ethane feed) to less than 30 weight percent ethylene (for naphtha and gas oil feeds). The higher boiling point feeds exhibit greater fouling tendencies in the pyrolysis furnaces, requiring additional furnace capacity to produce the same ethylene volume, and produce a greater yield of coproducts per yield of ethylene, requiring additional capacity in the reaction quench and separation section downstream of the pyrolysis furnaces.
The qualities desirable for the production of ethylene, propylene and higher-valued coproducts from olefin feedstocks (such as hydrogen content) generally decrease with increasing boiling point and undesirable qualities (sulfur, nitrogen, metals, polynuclear aromatics and asphaltene content) generally increase with increasing boiling point.
Olefin units capable of feeding naphthas and/or gas oils are relatively common, depending on local feedstock price and availability and coproduct value and demand. Olefin units capable of feeding higher boiling point streams (having a boiling point with the general range of 340.degree. C. to about 565.degree. C.) are also known.
U.S. Pat. No. 3,781,195 to Davis discloses a method for hydrotreating distillates having a boiling point of between 300.degree. to 650.degree. C. prior to subjecting the distillate to thermal cracking with steam at 700.degree. to 1000.degree. C. The distillates are prepared by vacuum flash distillation, with the highest boiling portion of the vacuum tower feed ("vacuum residuum") being rejected from the flash distillate. Asphaltenes which do not thermally crack but instead deposit as coke on the cracking furnace tubes, are removed in the vacuum residuum, along with a portion of the vacuum tower feed which would thermally crack to produce desirable olefins. The vacuum residuum is generally sold at a fuel value. Davis further discusses pretreatment of the distillate feedstock with hydrogen in the presence of a catalyst in order to reduce the content of aromatics, sulfur, nitrogen and metal compounds.
U.S. Pat. No. 4,257,871 to Wernicke discloses a process for the production of olefins by first deasphalting a vacuum residue and then, prior to hydrogenation, blending the deasphalted vacuum residue with a vacuum gas oil. The hydrotreatment employed in this process is known in the industry as hydrocracking. Hydrocracking produces a high yield of lower boiling distillates which are generally sold into the refined product fuels market. Only about 20 percent of the hydrogenated product has a boiling point in excess of 340.degree. C. The noted particularly active hydrogenation catalyst disclosed in Wernicke contains silica that is used to promote hydrocracking by providing acid sites for these reactions. Hydrocracking is defined as the breaking of carbon to carbon single bonds that are then saturated with hydrogen. These reactions primarily occur at tertiary carbon sites present in saturated polynaphthene hydrocarbons and less frequently at secondary carbon sites present in linear or paraffinic hydrocarbons. Hydrocracking will not typically occur at carbon to carbon double or triple bonds.